1. Field of the Invention
The present invention relates to a control apparatus and method for a machine tool, comprising opposed workpiece holding structures, for machining a workpiece held by the workpiece holding structures.
2. Description of the Background Art
Currently, machine tools such as a combined lathe, which holds a long workpiece by means of opposed workpiece holding mechanisms or structures, e.g., headstocks, and turns the workpiece while simultaneously driving the headstocks, are put to practical use. The use of a combined lathe is necessary where the workpiece is long and would twist or deform if driven only at one end.
FIG. 9 is an arrangement diagram showing a conventional combined lathe, as disclosed in Japanese Laid-Open Patent Publication No. HEI4-65701 wherein the numeral 1 indicates a tool rest, 2 denotes a cutting tool fitted to the tool rest 1, 3 designates a ballscrew coupled with the tool rest 1 for driving the tool rest, 4 represents an X-axis servo motor coupled with the ballscrew 3 for driving the ball screw, 5 indicates a workpiece, 11 designates a chuck for gripping one end of the workpiece 5, 12 denotes a headstock mounted with a spindle, 13 represents a ballscrew coupled with the headstock 12 for driving the headstock, and 14 indicates a Z-axis servo motor coupled with the ballscrew 13 for driving the ballscrew. There also are identical units 21-24, which correspond to the chuck 11, headstock 12, ballscrew 13 and Z-axis servo motor 14, and therefore will not be described.
This combined lathe turns the single workpiece 5 which is gripped at both ends by the chucks 11, 21. The lathe controls the movement of the headstocks 12, 22 simultaneously, in synchronization with each other.
FIG. 10 is a block diagram of servo amplifiers for a control apparatus, e.g., a numerical control apparatus (hereinafter referred to as the "NC"), for controlling the lathe shown in FIG. 9. In FIG. 10, an X-axis servo motor 4 is used for driving the tool rest 1. A position detector 6 detects the position of the tool rest 1, and an error counter 7 determines the difference between a position command pulse CPX from the NC and a feedback pulse from the position detector 6. A digital-to-analog converter 8 converts the value of the error counter 7 into an analog value, and a power amplifier 9 amplifies the analog value output by the digital-to-analog converter 8 to drive the X-axis servo motor 4.
A second group of components 16 to 19 and a third group of components 26 to 29 correspond respectively to the position detector 6, error counter 7, digital-to-analog converter 8 and power amplifier 9 to drive the first Z-axis servo motor 14 and the second Z-axis servo motor 24, respectively.
CPZ is a Z-axis position command pulse given by the NC to drive the first Z-axis servo motor 14 and the second Z-axis servo motor 24 at the same time.
Operation will now be described. The X-axis direction travel of the tool rest 1 and the Z-axis direction travels of the headstocks 12, 22 in FIG. 9 can be achieved by executing a turning program stored on a paper tape (not shown) or in memory or the like in the NC. In the turning program, the movement along the X axis and Z axes has been programmed on a block-by-block basis in order of execution, e.g.:
______________________________________ N001 G01X100. Z200. F2., N002 GC0Z-50.; . . . . ______________________________________
These instructions result in a calculation of a corresponding axis travel-per-block by a central processor constituted by a CPU and memory not shown (in the NC unit). The travel-per-block is converted into corresponding axis position command pulse trains, such as the X-axis position command pulse train CPX and the Z-axis position command pulse train CPZ shown in FIG. 10, by a conventional pulse distributor.
The X-axis position command pulse train CPX is added to the contents of the error counter 7 and a difference between the pulse train and a feedback pulse, which is negatively-fed back from the position detector 6, is given to the power amplifier 9 through the digital-to-analog converter 8 to drive the servo motor 4 at the speed corresponding to the error value, thereby moving the tool rest 1.
The Z-axis position command pulse train CPZ is sent in a similar manner with the exception that it is given to both of the error counters 17, 27 to operate the two headstocks 12, 22 shown in FIG. 9 in synchronization with each other.
FIGS. 11(a) and 11(b) show the influence of the displacements of a mechanical system and a workpiece itself on the machine and the workpiece, wherein the continuous lines indicate the machine and the workpiece before displacement occurrence and the broken lines represent those after displacement occurrence.
FIG. 11(a) shows an example wherein the displacement is compensated for by the deformation of the workpiece 5 when the rigidity of the workpiece 5 is smaller than machine rigidity/servo rigidity. FIG. 11(b) shows an example wherein the displacement is compensated for by the deformation of the machine when the rigidity of the machine is smaller than workpiece rigidity/servo rigidity. Where the rigidity of the servo is smaller than workpiece rigidity/machine rigidity, the motor torque is saturated and control cannot be exercised, thereby bringing the motors or drive amplifiers to a stop because of overload.
Since the conventional workpiece holding method for a machine tool used chucks to secure a workpiece as described above, the shapes of the chucks had to be matched with the shape of the workpiece. Also, the portions of the workpiece that were secured by the chucks could not be machined, and/or the workpiece may be marred by chucking force.
Another workpiece holding method in Japanese Laid-Open Patent Publication No. HEI4-69103 uses face drivers. In this conventional approach, an example wherein a workpiece is gripped by the face drivers and tail spindles is disclosed, as seen in FIG. 12. In FIG. 12, a tool rest 1 is arranged on a machine body 100 of a lathe so as to be movable in Z-axis directions of arrows A, B (the axial direction of the work spindle) and in X-axis directions of arrows G, H. A cutting tool 2 is mounted on tool rest 1.
On machine body 100, a headstock 101 is disposed on the left of tool rest 1 and a tailstock 102, on the right of tool rest 1 as viewed in FIG. 12. Further, on machine body 100 is a temporary holding stand 103, which is vertically movable in directions of arrows C, D through a pneumatic cylinder 104. Temporary holding stand 103 is interposed between headstock 101 and tailstock 102.
On headstock 101 is a cylindrical work spindle 105, which is rotatable in directions of arrows E, F. On a distal end of work spindle 105 is a chuck 106, which can rotate in directions of arrows E, F integrally with work spindle 105. A chuck gripper 107 is arranged on chuck 106 so as to be opened and closed through a drive cylinder 108 and an operation pipe 109.
Inside chuck 106 is a face driver 110, which can be advanced and retreated in directions of arrows A1, B1, i.e., the Z-axis direction with respect to chuck 106, through a drive cylinder 111 and an operation rod 112 in such a manner as to rotate in the directions of arrows E, F integrally with work spindle 105.
A tail spindle 113 is disposed on tailstock 102 so as to be movable in directions of arrows A2, B2, i.e., the Z-axis direction, through a sleeve 114.
A method of machining a crank shaft 115, which is a workpiece, will be described with reference to FIG. 12. Chuck gripper 107 is open before machining. First, crank shaft 115 is carried on temporary holding stand 103, which is thereafter moved up in the direction of arrow C. Then, operation rod 112 and face driver 110 are moved in the direction of arrow B1, and face driver 110 is fed to an advance position that is ahead of the chuck rightward. Then, tail spindle 113 is moved in the direction of arrow A2 to hold both end surfaces of crank shaft 115 with face driver 110 and tail spindle 113, and temporary holding stand 103 is thereafter lowered in the direction of arrow D. The outer circumference of crank shaft 115 is machined under this condition. Upon completion of the machining of the outer circumference, tail spindle 113 is moved in the direction of arrow A2, whereas face driver 110 is moved in the direction of arrow A1. With the machined outer circumference so moved as to be inserted into chuck gripper 107 that is in open position, chuck gripper 107 is closed. Crank shaft 115 is machined while held by chuck gripper 107 and tail spindle 113 thereafter. For a combined lathe which turns a long workpiece, however, the workpiece must be gripped by two opposed headstocks and the headstocks driven in synchronization with each other to turn the workpiece. Hence, the example has limitations.
Also, in the conventional apparatus constructed as described above, when the two headstocks are coupled by the workpiece, the displacement of the mechanical system and the displacement of the workpiece itself appear as loads on the servo motors. However, these displacements are the pressure displacement of the workpiece caused by chucking pressure, the thermal displacement of the workpiece attributable to heat generated during cutting, the thermal displacement of the machine due to the frictional heat of machine movement, and the like, and cannot be eliminated. For this reason, excessive force will be applied to the workpiece, reducing turning accuracy.
Known to improve the above disadvantages is Japanese Laid-Open Patent Publication No. HEI4-65701. A synchronous feed axis joint operation method for a lathe is disclosed in which the lathe has a first feed axis and a second feed axis for driving two opposed headstocks, the two headstocks are coupled via a workpiece, and the two feed axes are controlled to operate jointly in synchronization with each other. From a difference between the torques of the two feed axes under synchronous joint operation control, the displacement of the machine or the workpiece is detected and the compensation value of a position relative to the feed axes is calculated and used to compensate for the positions of the feed axes. This method is applicable to a case where both ends of the workpiece were held by chucks but could not be used for a case where both ends of the workpiece were held by face drivers because the face drivers must be kept pressed against the workpiece.